An increasing number of hardware attachments are available to expand upon the functional capabilities of mobile computing devices, such as smart phones. In some instances, card reader attachments, which enable the reading of payment information from a magnetic strip (such as a credit card), can be coupled to a host device via a standard 3.5 mm headphone port. In such a configuration, the payment information from the magnetic strip is forwarded to the host device by configuring the card reader to encode the payment information into an audio signal and transmit the audio signal to the host device via an audio input channel of the host device in the headphone port. The audio signal is then detected by the host device, decoded to retrieve the payment information, and the payment information is processed locally or remotely to effect payment. In a typical headphone port, the host device utilizes automatic gain control (AGC) circuitry to amplify signals to facilitate their detection. However, since AGC circuitry on the host device is typically not configured to discriminate between intentional and unintentional signals on the audio input channel, the host device may expend resources on processing signals other than those generated by the reader device.